This invention relates to twisted communication cables in general and more particularly to an improved method and apparatus for locating breaks in communication cables manufactured in a two stage twisting operation.
It is well known in the art to subject electrical cables and wires to a quality control test after manufacturing. Typically the cables are checked for interchanged conductors, conductor short circuits and conductor breaks. Particularly when testing communication cables, a large number of tests must be made. In addition to the types of tests noted above, tests to determine coupling properties of a cable are also necessary. To carry out such tests various production test equipment and methods have been developed to make possible to a large extent the automatic cycling of the individual tests. Typical are those methods and the apparatus disclosed in German Offenlegunsschrift 1,616,068 and German Auslegeschrift 1,947,569.
With one known method of determining interchanged, short circuited and interrupted conductors in multiconductor electrical cables, individual conductors of the cable are subdivided into a number of groups of series connected conductors and the terminal leads to these groups are each connected to a test relay. The switching states of the test relays are then indicated by indicator lamps. Such is described in German Auslegeschrift, 2,012,900.
In another prior art testing device optical means are used to automatically indicate faults such as interruption and conductor and ground shorts automatically. For this purpose the individual conductors are connected in series with test relays. Optical indicating elements for the type and location of faults and the simultaneous interruption of the test are controlled using contacts of the test relays. Such is disclosed in German Pat. No. 974,746.
The above described tests and methods have in common that the testing of the cables and wires takes place only after they are made, i.e., subsequent to the manufacturing process. The correction of faults which are located, e.g., the correction of the conductor break, thus necessitates a further operation in many cases, i.e., rewinding. In order to save an additional operation of this nature methods of testing during the fabrication of the cable, i.e., during the twisting of the cable conductors have been proposed. In such proposed methods the twisting process is stopped if deviations or faults are detected. For determining coupling in a twisted multiconductor communication cable made up of a number of pairs, for example, an arrangement for performing electrical production tests is known which makes possible production tests for all conductors of the cable during the fabrication. In this method corrections in the twisting operation such as a change in the location of the transposition point of the conductors can be carried out during the fabrication or can be performed automatically on the basis of an automatic evaluation of the measurement result. Such is disclosed in German Auslegeschrift 1,202,370. In another prior art arrangement, faults in the conductor insulation are determined during the twisting of the conductors with the detection of a fault resulting in an optical and/or acoustical indication and the stopping of the twisting process. In this arrangement which is disclosed in German Auslegeschrift 1,441,168 and a-c circuit which is closed through the ground capacities of the conductors is used as a measuring circuit.
The determination and correction of faults during the twisting of conductors into stranded units has not become an accepted practice because the correction of certain faults requires that the twisting process be stopped. As is well known such processes take place at high speed. Apart from the difficulty of localizing the fault accurately, stopping the twisting process causes a loss in production. Therefore up to this point it has been found more economical to perform the fault correction after the twisting operation is complete. That is this operation is carried as a separate operation particularly because of the relatively low frequency at which the faults occur.
Recently developed twisting techniques permit combining in a single operation twisting processes which were previously carried out separately. In particular, it is now possible to carry out the twisting of conductors to form stranded units such as pairs, triplets or spiral quads and then to twist several of such stranded units to form a twisted group also referred to as a base bundle all in one operation. Typically in these two stage twisting process at least the first twisting stage employs what is referred to as SZ twisting process in which twisting elements are twisted to form a stranded unit with alternating direction of twist. Clearly in such an operation the retwisting after the correction of faults can be a costly operation. As a result the need for a method and apparatus for carrying out fault detection during the twisting operation so that faults can be immediately corrected becomes evident.